Conventionally, ultrasonic diagnostic apparatuses have been widely used to perform observation and diagnosis of blood flow of a living body. An ultrasonic diagnostic apparatus performs generation and display of blood flow information from reflective waves of ultrasonic waves by Doppler method based on the Doppler effect. The blood flow information that is generated and displayed by the ultrasonic diagnostic apparatus includes a color Doppler image, a Doppler spectrum, and the like.
The color Doppler image is an ultrasonic image that is visualized by a color flow mapping (CFM). In CFM, transmission and reception of ultrasonic waves are performed on one scan line more than one time. Furthermore, in CFM, by applying a moving target indicator (MTI) filter to a data string at the same position, a signal (clutter signal) that is originated from a static tissue or a tissue that moves slowly is suppressed, and a signal that is originated from a blood flow is thereby extracted. In CFM, blood flow information such as a speed of blood flow, a dispersion of blood flow, and a power of blood flow is estimated from this blood flow signal, and an ultrasonic image (color Doppler image) in which a distribution of estimation results is displayed, for example, in two-dimensional color image is displayed.
Usually, as an MTI filter, a filter the coefficient of which is fixed, such as a Butterworth infinite impulse response (IIR) filter and a polynomial regression filter, is used. On the other hand, an adaptive MTI filter the coefficient of which is varied according to an input signal is also known.
As one example, an adaptive MTI filter acquires a speed of a tissue from a signal before input of the MTI filter, and acquires a signal in which the phase difference is cancelled. The filter selects a coefficient according to the acquired signal from among coefficients that have been prepared in advance for the MTI filter. Moreover, an adaptive MTI filter called “eigenvector regression filter” has also been known. This adaptive MTI filter acquires a signal in which a clutter component is suppressed by a method in which an eigenvector is calculated from a correlation matrix, and a coefficient to be used for the MTI filter is directly calculated from the calculated eigenvector. This method is an applied method of a technique used in principal component analysis, a Karhunen-Loeve transform, and a characteristic space method.
In such a conventional ultrasonic color Doppler method, input data strings are often at regular intervals. The data string indicates a set of reflected wave data that is generated based on reflected waves received as a result of transmitting ultrasonic waves on the identical scan line. Furthermore, a data string being at regular intervals indicates acquisition of reflected wave data at regular intervals of time, by receiving reflected waves by transmitting ultrasonic waves to the identical scan line in such a manner that the transmission pulses are at regular intervals of time. For example, when transmission and reception of ultrasonic waves are performed for four times to one scan line, an interval of transmission and reception of ultrasonic waves between the first time and the second time, an interval of transmission and reception of ultrasonic waves between the second time and the third time, and an interval of transmission and reception of ultrasonic waves between the third time and the fourth time are equal to each other. Moreover, in the ultrasonic color Doppler method, a method applied when a data string is not at regular intervals is also disclosed. In this method, for example, clutter is approximated by performing the least square fitting of a polynomial to the data string that is not at regular intervals, and a blood flow signal is extracted by subtracting this approximation signal from an original signal. A case in which “a data string is not at regular intervals” indicates acquisition of reflected wave data that is not at regular intervals of time by receiving reflected waves by transmitting ultrasonic waves to the identical scan line in such a manner that transmission pulses are not at regular intervals of time. A “data string not being at regular interval” is described as a “data string being at irregular intervals” appropriately.